ution of auxin also appears to be responsible for the curvature response to lateral light
stimulation (phototropism; see Chapter 4).
Gravity perception in stems occurs in the starch sheath parenchyma tissues that run the
length of the hypocotyl (Fukaki et al. 1998). Lateral auxin transport then is believed to
occur in multiple cells along the hypocotyl, and the elevated levels of auxin on the lower
flank of the hypocotyl stimulate cellular elongation and allow upward growth (Blancaflor
and Masson 2003). In contrast, roots sense gravity very locally in the columella cells in
the root cap (Blancaflor et al. 1998), and auxin is redistributed from the root tip to the
lower side of the root after gravity stimulation, rather than being laterally transported
across the root (as reviewed by Blancaflor and Masson 2003; see Chapters 1 and 2). As
elevations in auxin concentration generally inhibit root growth, this redistribution would
result in slower growth on the lower side relative to the upper side, resulting in downward
root growth.
Asymmetric redistribution of radiolabeled IAA has been measured in both shoots
(Parker and Briggs 1990) and roots (Young et al. 1990), preceding differential gravi-
tropic growth (Parker and Briggs 1990). Additionally, gradients in endogenous free IAA
have been observed across gravity-stimulated oat and maize pulvini and maize coleop-
tiles (Kaufman et al. 1995; Philippar et al. 1999; Long et al. 2002). Growth of seedlings
on IAA efflux inhibitors (as reviewed in Rubery 1990) leads to a rapid inhibition of the
gravity response in a number of plant species under conditions where growth still occurs
48 PLANT TROPISMS
Figure 3.1. Auxin transport is polar in Arabidopsis and other plants. A. In an upright hypocotyl, inflores-
cence, and other stem tissues, auxin moves in single direction, from the shoot apex toward the base
(basipetal). In roots, movement from the shoot into the root is from the base toward the root apex (acropetal)
through cells in the central cylinder. In roots, auxin also moves from the root tip toward the base in a basipetal
direction through cells of the cortex and/or epidermis. B. In a plant reoriented 90 degrees relative to gravity,
although auxin transport continues in a polar fashion, lateral auxin transport also occurs. In shoot tissues, this
transport may occur across the hypocotyl, whereas in roots, redirection of auxin transport is believed to be
controlled from the root cap. The regions in which gravitropic bending will occur are also indicated.
This figure is reprinted from Muday (2001) with permission of Springer Publishing.